The invention relates to a plasma spray torch comprising a spray nozzle which forms an electrode and which includes a nozzle duct and which in particular is connected anodically, and a second electrode associated with the spray nozzle, in a torch portion, which is electrically insulated relative to the spray nozzle, of a torch arm which has flow ducts for working gas and for cooling agent which flows nozzle-wards in one of the flow ducts and which, after the cooling operation has taken place, is discharged from another flow duct, wherein a feed duct for powder opens into the nozzle duct. The invention also relates to a method of internally coating a tube.
An apparatus of that nature is proposed in German laid-open application (DE-OS) No. 34 30 383, for the production of internal coatings in holding grooves in turbine discs. That apparatus comprises a torch head with anode and cathode half-shell portions which can be pivoted away from each other; carried in the latter half-shell portion is an electrode which projects into the nozzle duct of a spray nozzle in the anodic portion of the torch head. The direction of spray in that arrangement is at right angles to the axis of the torch head, while the feed of powder is very closely adjacent to the electrode, directly at the wall of the nozzle duct.
For cooling purposes, the arrangement has nozzle openings on a nozzle ring which is disposed in a support-like manner around the torch head at a spacing relative to the nozzle duct; the above-mentioned openings provide for the production, for cooling purposes, of a gas protective envelope which is also intended to blow spray dust and powder away.
The previous arrangement is essentially restricted to a rotationally symmetrical configuration of the electrode head which projects into a torch nozzle which is of a particular configuration in terms of cross-section.
Having regard to that art, the inventor set himself the object of improving a plasma spray torch of the kind set forth in the opening part of this specification, for the internal coating of very narrow tubes or the like cavities or hollow spaces, in terms of its mode of operation, and in particular controlling the adjustability of its arc and the relationship thereof relative to the fusion zone, and optimizing the cooling action. In addition the invention seeks to provide that the construction of the plasma spray torch is of a completely different configuration which also simplifies access to the individual components.